A ring laser gyroscope (RLG) utilizes a laser beam directed to travel in a closed path (that is, a ring) to detect rotation about the axis of the path around which a laser beam is directed. The laser beam is directed in its path by mirrors, typically in a triangular path having three mirrors (with each mirror located at a corner of the triangular path). The RLG is capable of operating over a wide range of temperatures. Frequently, internal elements of the RLG suffer from thermal expansion and contraction due to temperature changes. As a result, these temperature changes expand or contract the internal elements and unless compensated for will cause a change in the path length.
For proper operation, referred to here as a target mode of operation, the RLG requires a laser path maintained at a substantially constant length. The RLG is most accurate when operating in the same target mode determined during calibration. In order to maintain a constant ring laser path length, mirror transducers are commonly employed. Typically, a servo loop is used to control the mirror transducers in order to compensate for the thermal expansion effects which cause the undesirable path length variations. The accuracy of an RLG significantly relies upon the ability to compensate for changes in the total path length and to retain a substantially similar length (in the geometric sense) to the original (that is, the calibrated) path length.
Mode shifts occur when path length control fails to repetitively retain a substantially similar total laser path length compared to the original calibrated path length (for example, after a power interruption the RLG attains and operates at a different integer number of wavelengths from where it operated during the calibration process). These mode shifts result in a discrete RLG scale factor shift. For the purposes of this description, an RLG scale factor is defined as the ratio of actual angle rotated about the gyros input axis to the gyro reported output. For example, typical RLG scale factor units include arc-seconds per count. The RLG scale factor will change a discrete amount for each integer wavelength change in total path length: the scale factor decreases as the total path length increases, and the scale factor increases when the total path length decreases.
To ensure mission success, each RLG system application must operate within specified accuracy requirements. Some missions require very accurate RLG scale factor performance, and are therefore intolerant of the change in scale factor which results from even a single mode shift. Such a project must typically implement frequent guidance system pull and re-calibration intervals in order to ensure accurate scale factor performance. However, the removal of an RLG-based system from a launch vehicle (for example, a spacecraft) for frequent calibrations can lead to significant downtime while the launch vehicle is unavailable. In addition, these frequent calibrations increase product safety and handling costs and system life cycle costs.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improvements in RLG-based guidance systems which require precision scale factor performance.